Parasitic power losses are a concern with closed loop hydraulic systems, including, but not limited to, hydrostatic vehicle proportion drives and differential steering systems. It has been found that a major contributor to such parasitic losses in these systems is the wasted hydraulic power of the charge flow being throttled across a charge relief valve. Charge systems typically provide power proportional to engine power at a constant pressure for system oil makeup and control actuation. Under some operating conditions, the charge system flow is substantially greater than that required. For instance, at a null condition, that is, wherein the main pump is not providing flow, charge flow is only needed to provide makeup for leakage and/or flushing in the closed loop system. Thus, it has been observed that at conditions such as the null condition the charge system flow can be significantly reduced. However, it has previously been problematic to reduce charge flow in a closed loop system without incurring corresponding parasitic power losses.
Accordingly, the present invention is directed to achieving one or more of the objectives as set forth above.